The conductivity of a polymer semiconductor, poly[2-methoxy-5-(2’-ethylhexyloxy) -p-phenylene vinylene] (MEH-PPV) was drastically improved by n-type doping with LiF. The improvement of the conductivity is mainly caused by the improved carrier mobility. This phenomenon is attributed to trap filling by doping. The carrier generation should take place at a trap level of the semiconductor and the trap level can be filled, resulting in carrier mobility improvement. Furthermore, we fabricated the organic Schottky solar cells with n-doped MEH-PPV. Open circuit voltage (Voc) was significantly increased as the doping concentration increased, indicating that LiF in MEH-PPV can change Fermi level.